1. Field of the Invention
The present invention relates in general to a ceramic rotor and metal shaft assembly for use in turbochargers, gas turbines, etc. and more particularly to a ceramic rotor and metal shaft assembly of the kind that the ceramic rotor and metal shaft are joined together at their protruded and recessed portions by making the recessed portion tightly or firmly hold the protruded portion.
2. Description of the Prior Art
The use of a ceramic turbine wheel in a turbocharger with a view to improving the responsiveness is well known in the art. The shaft for supporting the ceramic turbine wheel is made of metal, and the ceramic turbine wheel and the metal shaft are joined by for example shrinkage fit. The shrinkage fit is a method of joining a protruded portion of the turbine wheel to a recessed portion of the shaft by heating the recessed portion up to a predetermined high temperature and then coupling the protruded and recessed portions together so that by the effect of shrinkage of the recessed portion when cooled the protruded and recessed portions are firmly joined together.
For the purpose of attaining shrinkage fit, the coefficient of thermal expansion of the recessed portion made of metal is set larger than that of the protruded portion made of ceramics. Due to this, after shrinkage fit, tensile stress arises at the joint between the recessed and protruded portions, thus lowering the strength of the joint between the turbine wheel and metal shaft.
When the strength of the joint between the turbine wheel and shaft is lowered, a variation of the rotational balance of the turbine wheel may possibly be caused after a long period of usage.
In order to overcome this problem, it has heretofore been practiced to provide a groove to the outer periphery of the recessed portion. This however causes a large design restriction.
To the same end, it is also known to provide between the recessed and protruded portions a brazing metal which serves as a buffer member. However, due to the necessities of a plating process for attaining uniform formation of a brazing metal on the inner circumferential surface of the recessed portion and the outer circumferential surface of the protruded portion, a process of removing unnecessary plating, etc., a high manufacturing cost results.
It has further been proposed to provide a clearance between the bottom surface of the recessed portion and the axial end of the protruded portion. This however is encountered by another problem that it is difficult to control variation of the clearance from product to product within a predetermined allowance.